1. Field of the Invention
The present invention relates to the preparation of adipic acid by hydrocarboxylation of pentenoic acids, and, more especially, by reacting water and carbon monoxide with at least one pentenoic acid, in the presence of a rhodium-based catalyst, as well as at least one iodinated promoter therefor.
2. Description of the Prior Art
Published European Patent Application No. 188,209 describes a process for preparing linear dicarboxylic acids, in particular adipic acid, by reaction of unsaturated monocarboxylic acids, in particular penten-3-oic acid, carbon monoxide and water, in the presence of a rhodium-based catalyst and of an iodinated promoter, the reaction being carried out in a solvent such as methylene chloride at a temperature of 100.degree. to 240.degree. C. and under a total pressure of between 14 and 240 atm; a temperature of between 150.degree. and 180.degree. C. and a total pressure of between 24 and 40 atmospheres are indicated as preferable. The carbon monoxide partial pressure is generally between 10 and 35 atmospheres and preferably between 10 and 17 atmospheres. The selection of the solvent is considered to be critical for the process described and it is even indicated that solvents such as acetic acid are undesirable due to the low degrees of linearity obtained in their presence.
Likewise, it is maintained that nonpolar solvents such as cyclohexane and toluene are also undesirable, due to their propensity to directly promote the formation of branched compounds and, indirectly, saturated monocarboxylic acids.
Published European Patent Application No. 0,274,076 describes a process for preparing linear carboxylic acids by hydroxycarboxylation of unsaturated esters or of terminally unsaturated alkenes comprising from 4 to 16 carbon atoms, in the presence of a rhodium-based catalyst and of an iodinated promoter. The reaction is carried out in a solvent indiscriminately selected from among methylene chloride, 1,2-dichloroethane and aromatic solvents. An aliphatic or aromatic acid having a pK.sub.a of between 4.2 and 5.2 is present as the reaction accelerator. The carbon monoxide partial pressure is between 10 and 200 atmospheres and preferably between 13 and 20 atmospheres.
However, when using pentenoic ester starting materials, the formation of methyl monoadipate is essentially observed.
EP-A-0,477,112 describes the preparation of adipic acid by hydrocarboxylation of pentenoic acids in the presence of rhodium and of an iodinated promoter in a solvent selected from among the carboxylic acids.
In sum, all of the processes of the prior art are carried out in a solvent of one type or another.